HELP! I made a mistake when sculpting and when trying to fix it, it lead to a bigger problem
There seems to be an easy fix for your particular mesh problem: (because it is a nice flat internal surface)


Go into front view and turn off perspective (press 1 and then 5 on Numpad, ... or tick emulate numpad in the preferences menu if you don't have one, in that case also 1, 5)
Its always very important to see what is going wrong without all the rest blocking the view, so then use Alt+B to box-restrict the visible mesh around the center:
Then select one of the internal faces and select "linked flat faces" (shortcut: shift+ctrl+alt+F)
then delete those with ->delete->Faces and check if this solved the problem. As you can see in the right picture in your particular case the mesh is already clean after this.

Now you can use Alt+B again to return to the normal unrestricted view.


That should have done it! Happy blending! :)